netdev - Re: [next-queue PATCH v1] igb: Fix limiting the number of queues to number of cpus

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Date:   Thu, 04 Apr 2019 17:04:36 -0700
From:   Jeff Kirsher <jeffrey.t.kirsher@...el.com>
To:     Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        intel-wired-lan@...ts.osuosl.org
Cc:     netdev@...r.kernel.org,
        Bhagath Singh Karunakaran <bhagath@...ycito.com>
Subject: Re: [next-queue PATCH v1] igb: Fix limiting the number of queues to
 number of cpus

On Thu, 2019-04-04 at 14:56 -0700, Vinicius Costa Gomes wrote:
> We have seen some reports[1] of users complaining that they aren't
> able to use some queues when their machines have less than 4 cpus.
> This affects some TSN workloads, as different traffic classes are
> assigned different queues. The current behavior limits the number of
> traffic classes that can be reliably handled.
> 
> In practice, what is not working, it returns an invalid parameter
> error, in hosts with less than 4 cpus is something like this:
> 
> $ tc qdisc replace dev IFACE parent root mqprio \
>            num_tc 3 map 2 2 1 0 2 2 2 2 2 2 2 2 2 2 2 2 \
>            queues 1@0 1@1 2@2 hw 0
> 
> Because changing the default logic of the allocation of queues could
> bring other effects, we propose adding a module parameter so expert
> users may override that decision.
> 
> [1] https://github.com/jeez/iproute2/issues/1
> 
> Reported-by: Bhagath Singh Karunakaran <bhagath@...ycito.com>
> Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@...el.com>
> ---
> 
> A similar fix should also be needed for igc, even if I don't have the
> hardware to test, I can produce a patch, if others are able to test.
> 
> I am not totally sure that using a module parameter is the best
> solution, so, suggestions are welcome.
> 
>  drivers/net/ethernet/intel/igb/igb_main.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)

A module parameter maybe fine for our out-of-tree driver, but not for the
kernel driver.

NACK on the basis that a new module parameter is being introduced for the
driver.  This is not acceptable by Dave Miller or myself.  As of now, I do
not have a alternative solution to propose unfortunately.

I will discuss the issue with my fellow developers and hopefully we can
come up with a kernel interface that all drivers can use to handle this
issue.

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